Reduced flicker AC LED lamp with separately shortable sections of an LED string

Abstract

An LED lamp with an integrated circuit, a rectifier, and a string of series-connected LEDs rectifies an incoming AC signal. The integrated circuit includes power switches that can separately and selectably short out a corresponding one of several groups of LEDs in an LED string across which the rectified AC signal is present. As the voltage across the string increases, the integrated circuit controls the power switches to increase the number of LEDs through which current flows, whereas as the voltage across the string decreases the integrated circuit controls the power switches to decrease the number of LEDs through which current flows. The flow of LED string current is broken to reduce flicker. Alternatively, a valley fill capacitor peaks LED current during the valleys of the incoming AC signal to reduce flicker. LED current is regulated to provide superior efficiency, reliability, power-factor correction, and lamp over-voltage, -current, and -temperature protection.

What is claimed is: 
     
       1. A method comprising:
 increasing a number of light emitting diodes (LEDs) in a string of series-connected LEDs through which an LED current flows from a first node to a second node as a voltage amplitude of a changing voltage signal increases to a peak amplitude, wherein the number has at least two discrete nonzero values as the voltage amplitude of the changing voltage signal increases, and wherein the changing voltage signal has a cycle period; 
 decreasing the number of LEDs in the string of series-connected LEDs through which the LED current flows from the first node to the second node as the voltage amplitude of the changing voltage signal decreases from the peak amplitude, wherein the changing voltage signal has an associated current magnitude; and 
 reducing the LED current during the cycle period by more than half of the current magnitude of the changing voltage signal for a duration of less than 12% of the cycle period. 
 
     
     
       2. The method of  claim 1 , wherein the periodic peak amplitude of the changing input voltage signal occurs about every 8.33 milliseconds, and wherein the duration of reduced LED current is between 0.7 and 0.9 milliseconds. 
     
     
       3. The method of  claim 1 , wherein the changing voltage signal is generated by rectifying a sinusoidal alternating current signal, wherein the sinusoidal alternating current signal has a frequency, and wherein the reducing the LED current is performed such that peaks of light intensity are emitted from the string of series-connected LEDs at a rate that is four times the frequency of the sinusoidal alternating current signal. 
     
     
       4. The method of  claim 1 , wherein the voltage signal is generated by rectifying a sinusoidal alternating current signal, and wherein the reducing the LED current is performed such that peaks of light intensity are emitted from the string of series-connected LEDs at more then 130 times per second. 
     
     
       5. A method comprising:
 rectifying a sinusoidal alternating current (AC) voltage signal having an amplitude higher than one hundred volts to generate a changing voltage signal, wherein the changing voltage signal has a period and instantaneous voltages; 
 supplying a string of series-connected light emitting diodes (LEDs) with the changing voltage signal, wherein the string of series-connected LEDs through which an LED current flows has a total forward voltage drop that approximately equals the instantaneous voltage during a portion of the period of the changing input voltage signal; and 
 modulating light that is emitted from the string of series-connected LEDs such that peaks of light intensity are generated at more than 130 times per second, wherein the string of series-connected LEDs is continuously supplied with the LED current such that the peaks of light intensity are generated from continuous LED power. 
 
     
     
       6. The method of  claim 5 , wherein the sinusoidal AC voltage signal has a frequency, and wherein the peaks of light intensity are generated at a rate that is four times the frequency of the sinusoidal AC voltage signal. 
     
     
       7. The method of  claim 5 , wherein the changing voltage signal has a peak amplitude, and wherein the modulating involves reducing the LED current when the changing voltage signal is at or near its peak amplitude. 
     
     
       8. A system comprising:
 a first terminal of an integrated circuit, wherein a first node is coupled to the first terminal, wherein a changing voltage signal with a voltage amplitude and a cycle period is present on the first node, and wherein the changing voltage signal has an associated current magnitude; 
 a second terminal of the integrated circuit, wherein a second node is coupled to the second terminal; and 
 a string of series-connected LEDs through which an LED current flows from the first node to the second node, wherein a number of light emitting diodes (LEDs) in the string of series-connected LEDs through which the LED current flows is increased as the voltage amplitude of the changing input voltage signal increases to a peak amplitude, wherein the number of LEDs in the string of series-connected LEDs through which the LED current flows is decreased as the voltage amplitude of the changing input voltage signal decreases from the peak amplitude, wherein the integrated circuit reduces the LED current during the cycle period by more than half of the current magnitude of the changing voltage signal for a duration of less than 12% of the cycle period. 
 
     
     
       9. The system of  claim 8 , wherein the duration of reduced LED current occurs from before the voltage amplitude of the changing voltage signal reaches the peak amplitude at least until the changing voltage signal is at its peak amplitude. 
     
     
       10. The system of  claim 8 , wherein the integrated circuit adjusts the LED current based on a dimmer phase cut angle. 
     
     
       11. The system of  claim 8 , wherein the changing voltage signal is generated by rectifying a sinusoidal alternating current signal, and wherein the integrated circuit reduces the LED current such that peaks of light intensity are generated at more than 130 times per second. 
     
     
       12. The method of  claim 5 , further comprising:
 adjusting the LED current based on a dimmer phase cut angle. 
 
     
     
       13. The method of  claim 12 , wherein the LED current is adjusted so that the greater the dimmer phase cut angle, the lower the average of the continuous LED power becomes.